LED based lighting devices, or LED lamps, have become common on the market and are showing great promise to gradually replace incandescent and compact fluorescent lamps throughout the world due to long life-time expectancy, reduced size, and high energy-efficiency with respect to energy and lumen output efficiency as compared to for instance traditional incandescent light bulbs. Utilizing LED based lamps in traffic lights, a city can significantly reduce the energy related cost per year per signal, because a LED lamp uses approximately one-tenth of the electricity that the traditionally used illumination does.
Thermal management of LED lamps is key, since the performance of the LED lamp is often limited in the light output by thermal constraints. Thermal management may be concerned with managing heat produced by the LED lamp itself, as well as external heat sources, or may be related to influence on the LED lamp by the ambient temperature. Generally, the thermal performance determines the maximum light output from the LED lamp, and is further determined mainly by the size of the heated external surface of the LED lamp. As an example, consider a typical retrofit LED lamp comprising at least one LED-based light source arranged in thermal contact with a heat sink, i.e. typically the lamp base. The LED-based light source is arranged for generating light which exits the LED lamp through a light exit element, i.e. an optically transmissive element like for instance a bulb envelope. The light exit element is typically made of a transparent or translucent material, like glass, silicone, and Polycarbonate, PC, which materials all have a low thermal conductance. Therefore heat spreading from the heat sink into the bulb envelope is not effective, and most of the heat produced by the LEDs therefore exits the lighting device via the heat sink.
It is known in the art to increase the heated external surface of the LED lamp by means of providing heat spreading from the heat sink to the light exit element. WO2010/097721 A1 discloses a LED lamp including a LED-based light source configured to emit light and an optically transmissive window optically and thermally coupled to the LED-based light source. Different solutions for configuring the optically transmissive window to in an improved manner radiate heat generated by the LED-based light source to the ambient, as compared to the typical prior art LED lamp as described above, are shown. For instance, the document discloses the optically transmissive window being arranged with one of a coating with predetermined heat conductivity, a compound material, an at least partly integral heat pipe, and a combination of elements including two materials with different thermal conductivities.